GAS weighs 8lbs a gallon.
Weight reduces gas mileage.
So i would think it is best to drive with as little gas in the tank as possible.
LEts say you have a 12 gallon tank and fill up to full tank.That fuel weighs 96lbs...If you fill up with only 4 gallons youve just reduced your weight by 64lbs.
Thoughts?

Updated the post above, but:
My car sputters and really doesn't want to start/restart when it's low. I do a lot more engine-on coasting and leave it running at stops instead of shutting down. There's still fuel available, but there's something it doesn't like. It's probably an issue with my car.